According to Federal Aviation Administration (FAA) regulations, fuel lines (e.g., manifolds, conduits) that conduct or transport fuel through areas classified by the FAA as “ignition zones” (e.g., pressurized areas) must be shrouded and capable of being drained. The applicable sections of the FAA requirements relative to fuel lines in aircraft ignition zones for transport category airplanes are contained in 14 C.F.R. Part 25, the pertinent parts of which are set forth below.
For example, Section 25.855 states in pertinent part that “for each cargo and baggage compartment not occupied by crew or passengers . . . no compartment may contain any controls, wiring, lines, equipment, or accessories whose damage or failure would affect safe operation, unless those items are protected so that . . . they cannot be damaged by the movement of cargo in the compartment, and . . . their breakage or failure will not create a fire hazard.” Section 25.863 states in pertinent part that “in each area where flammable fluids or vapors might escape by leakage of a fluid system, there must be means to minimize the probability of ignition of the fluids and vapors, and the resultant hazards if ignition does occur.” Section 25.901 states in pertinent part that “for each power plant and auxiliary power unit installation, it must be established that no single failure or malfunction or probable combination of failures will jeopardize the safe operation of the airplane except that the failure of structural elements need not be considered if the probability of such failure is extremely remote.” Section 25.967 states in pertinent part that “each fuel tank must be isolated from personnel compartments by a fumeproof and fuelproof enclosure.”
To summarize, the FAA regulations ensure that no single failure will jeopardize safe operation of the aircraft, as would be the case if single-wall fuel manifolds were used in aircraft “ignition zones.” That is, a leak or rupture in a single-wall fuel manifold would allow fuel to enter the “ignition zone”, which could result in catastrophic event were the fuel to ignite or explode.
To comply with the FAA shrouded fuel line requirements, several methods have been employed. One method involves the installation of a hose inside a tube. To be compatible with the present aerial refueling manifold which includes a five-inch (5.0″) outside diameter duct, this concept requires the hose to be disposed within a twelve inch (12.0″) outside diameter tube. Due to space limitations aboard aircraft, however, the relatively large outside diameter required for the outer tube has rendered this hose-in-tube concept impracticable.
In another effort at least in part to satisfy the FM shrouded fuel line requirements, an alternative method has involved placing an inner tube in a second tube without any means for supporting the inner tube. Accordingly, the inner tube in this concept is unable to transfer loads to the outer tube and/or to components external thereto and is thus not viable.